The present invention relates to electrochromic display devices utilizing the electrochromism effect which is a reversible coloring or light emitting phenomenon of a material due to electrical action. More particularly, the invention relates to a reflection type electrochromic display device which provides an excellent display background.
As is generally known, an electrochromic display device is made up of a display electrode, a confronting electrode, and an electrolyte. The electrochromic material is provided in the form of a film on a transparent conductive layer either as a part of the display electrode or dissolved in the electrolyte.
Examples of the electrochromic materials known in the art are inorganic electrochromic materials such as tungsten oxide, molybdenum oxide and silver iodide, and organic electrochromic materials such as compounds of viologens. The electrolyte employed is prepared by adding a mixture of sulfuric acid and glycerol or electrolyte such as lithium perchlorate or potassium bromide to a solvent such as water, propylene carbonate, cellosolve acetate, carbitol acetate, or .gamma.-butyrolactone.
The electrochromic display devices are classified into transmission and reflection type devices. In general, the reflection type electrochromic display device is made by colored electrochromic material on a background. In order to provide this background, insoluble white material such as titanium oxide or barium sulfate is mixed and dispersed in an electrolyte. As a result of the mixing of such white material, when the electrochromic material is colored, the display background has a high contrast and the confronting electrode can be hidden. However, the material thus mixed and dispersed often disadvantageously separates over a long period of time or its concentration becomes non-uniform because of aggregation.
In the above-described method, the type and quantity of material to be mixed and dispersed are limited and therefore the thickness of the electrolyte-impregnated layer must be increased before it is applied to an actual electrochromic display device. In order to solve these problems, a method has been proposed in which a porous member is used as the electrolyte-impregnated layer to provide the display background. However, that method is still disadvantageous in that it is difficult to sufficiently hide the base, and the electrical resistance of the electrolyte-impregnated layer is increased excessively. Thus, no fully acceptable electrolyte-impregnated layer for the display background has hitherto been known.